A subdomain is a website address, which is under a main, or a root domain. An example is something.domain.com, in which domain.com is the main domain address, while "something" is the subdomain below it. Every single subdomain can have its own site and records i.e. apart from employing the domain name as part of the whole web address, a subdomain can be entirely independent and even hosted by a different company. Using a subdomain can be extremely handy if you'd like to have different websites which are associated in some way. As an example, you may have a web-based store under the main domain and a forum under a subdomain so as to offer means for your customers to ask questions and share thoughts about your products. The main advantage of employing a subdomain as opposed to a subfolder is that the websites are going to be separated, so you can update one of them or perform maintenance not affecting the other.